In electrophotographic applications such as xerography, a charge retentive surface is electrostatically charged, and exposed to a light pattern of an original image to be reproduced to selectively discharge the surface in accordance therewith. The resulting pattern of charged and discharged areas on that surface form an electrostatic charge pattern (an electrostatic latent image) conforming to the original image. The latent image is developed by contacting it with a finely divided electrostatically attractable powder referred to as "toner". Toner is held on the image areas by the electrostatic charge on the surface. Thus, a toner image is produced in conformity with alight image of the original being reproduced. The toner image may then be transferred to a substrate (e.g., paper), and the image affixed thereto to form a permanent record of the image to be reproduced. Subsequent to development, excess toner left on the charge retentive surface is cleaned from the surface. The process is well known, and useful for light lens copying from an original, and printing applications from electronically generated or stored originals, where a charged surface may be imagewise discharged in a variety of ways.
Although a preponderance of the toner forming the image is transferred to the paper during transfer, some toner invariably remains on the charge retentive surface, it being held thereto by relatively high electrostatic and/or mechanical forces. Additionally, paper fibers, Kaolin and other debris have a tendency to be attracted to the charge retentive surface. It is essential for optimum operation that the toner remaining on the surface be cleaned thoroughly therefrom.
A commercially successful mode of cleaning employed in automatic xerography utilizes a brush with soft fiber bristles which have suitable triboelectric characteristics. While the bristles are soft they are sufficiently firm to remove residual toner particles from the charge retentive surface. For more effective cleaning, dual brush cleaning arrangements have been proposed, wherein a pair of brushes are located within a housing, as shown for example in U.S. Pat. Nos. 3,795,025 to Sadamitsu, 4,134,673 to Fisher, Xerox Disclosure Journal, Modified Dual Brush Cleaner, Donald J. Fisher, Vol. 2, No. 3, May/June 1977, pp. 85, 86. Dual brush arrangements are known for other purposes as well, such as for applying surface treatment materials to the a charge retentive surface, as shown in U.S. Pat. No. 4,622,914 to Garris. In a dual brush arrangement, flicker bars and directed air flow are used to remove toner from the brush rolls.
In certain electrophotographic processes, and particularly in highlight color applications that provide two types of toner to develop latent images on a surface, a phenomenon of filming is noted. Filming is characterized by the tenacious adherence of very fine toner material residues, toner additives and paper debris to the charge retentive surface. For reasons that are not clearly understood, brush cleaning is not an effective cleaning device in systems where a high degree of filming is present. Film cleaning can be improved in brush cleaners by stiffening the brush fibers, but film still remains on the surface.
To alleviate the filming problem, an abrasive surface or cutting edge is commonly used to remove the film. Accordingly, U.S. patent application No. 160,434 to Lindblad et al. filed Feb. 23, 1988, and assigned to the same assignee as the present invention, shows an auger arrangement providing porous foam or poromeric surfaces, or cutting edges for the removal of film buildup. U.S. Pat. No. 3,807,853 to Hudson, and U.S. Pat. No. 4,230,406 to Klett, for dry toner, while U.S. Pat. No. 4,436,054 to Ceelen et al. and U.S. Pat. No. 4,439,035 to Landa suggest the use of foam cleaning rolls for liquid toner. Dual foam rolls with porous surfaces are also proposed, as in U.S. Pat. No. 3,807,853 to Hudson, perhaps having different surface porosity or absorbing characteristics. Poromeric materials tend to collect toner in the pores of the material, which in movement past the charge retentive surface have the tendency to abrade or scour the surface. A light scouring or abrading action is desirable, but too heavy a scouring action will tend to damage coatings on the charge retentive surface. However, when a foam roll is arranged to provide an adequate cleaning function, it proves to be excessively abrasive, and may damage a soft photoconductive layer on a charge retentive surface.
Combination of other cleaning devices with foam rolls have been proposed, combining the abrasive properties of foam rolls with the better cleaning properties of other cleaning devices such as blades or brushes. Thus for example, Xerox Disclosure Journal, "Apparatus for Reducing Photoreceptor Filming", Thomas J. Kane, Vol. 2, No. 4, July/August 1977, p. 85, shows an foam roll preceding a brush cleaner, while U.S. Pat. No. 3,947,108 to Thettu suggests the foam roll following a blade. However, placing the foam roll preceding the fiber brush, as in U.S. Pat. No. 3,947,108 to Thettu, loads the roll with too much toner, creating the potential for too much abrasion or scouring. Use of the foam member after the blade, as in "Apparatus for Reducing Photoreceptor Filming", while providing appropriate abrasion, does not utilize the cleaning action of a roll cleaner in the cleaning arrangement.
Redundant cleaning systems are commonly used to improve cleaning quality by allowing a stressed primary system to operate in a less than perfect mode, and using a secondary system to make certain that the surface is completely cleaned. Thus, for example, U.S. Pat. No. 3,957,509 to McMullen et al. shows the combination of a blade preceding a stiff brush; U.S. Pat. No. 3,947,108 to Thettu suggests the foam roll following a blade; Xerox Disclosure Journal, "Apparatus for Reducing Photoreceptor Filming", Thomas J. Kane, Vol. 2, No. 4, July/August 1977, p. 85, shows a foam roll preceding a brush cleaner; U.S. Pat. No. 3,795,025 to Sadamitsu, U.S. Pat. No. 4,134,673 to Fisher, and Xerox Disclosure Journal, "Modified Dual Brush Cleaner", Donald J. Fisher, Vol. 2, No. 3, May/June 1977, pp. 85, 86, show dual brush arrangements; U.S. Pat. No. 3,807,853 to Hudson shows a dual foam roll arrangement.